1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of erasing data thereof.
2. Description of the Related Art
A NAND flash memory is conventionally known as a nonvolatile semiconductor memory. In the NAND flash memory, data erasure is performed at a time on each block that is a set of memory cells.
Various data erase sequences have been proposed. A technique is well known which performs soft programming and verification for the soft programming after an erase verify operation. The soft programming is an operation of programming a memory cell in an erased state to shift the threshold voltage of the memory cell in a positive direction.
In the conventional soft programming, all word lines in the same block are selected and all the memory cells in the same block are programmed at a time. Such a technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2007-305204.